1. Field of the Invention
This invention generally relates to gas-liquid contactors used in the removal of acidic gases, such as from utility and industrial flue gases. More particularly, this invention is directed to a gas-liquid contactor whose operation involves the total entrainment of a contact medium in a high-velocity flue gas so as to remove acidic constituents from the gas, followed by substantially complete removal of the entrained contact medium prior to demisting of the gases.
2. Description of the Prior Art
Gas-liquid contactors are widely used to remove substances such as acidic constituents and particulate matter from combustion or flue gases produced by utility and industrial plants. Often of particular concern are sulfur dioxide (SO.sub.2) and other acidic gases produced by the combustion of fossil fuels and various industrial operations. Acidic gases are known to be hazardous to the environment, such that their emission into the atmosphere is closely regulated by clean air statutes. The method by which acidic gases are removed with a gas-liquid contactor or other type of flue gas scrubber is known as wet flue gas desulfurization (FGD).
The cleansing action produced by gas-liquid contactors and absorbers is generally derived from the passage of gas through a tower cocurrently or countercurrently to a descending liquid that absorbs sulfur dioxide. Wet flue gas desulfurization processes have typically involved the use of an alkaline scrubbing liquid, such as a calcium-based slurry or a sodium-based or ammonia-based solution. As used herein, a slurry is a mixture of solids and liquids in which the content of the solids can be any desired level, including the extreme condition in which the slurry is termed a moist solid. Examples of calcium-based slurries are limestone (calcium carbonate; CaCO.sub.3) slurries and hydrated lime (calcium hydroxide; Ca(OH).sub.2) slurries formed by action of water on lime (calcium oxide; CaO). Such slurries react with the acidic gases to form precipitates that can be collected for disposal, recycling or sale. Intimate contact between the alkaline slurry and acidic gases that are present in the flue gases, such as sulfur dioxide, hydrogen chloride (HCl) and hydrogen fluoride (HF), result in the absorption of the gases by the slurry and the formation of salts, such as calcium sulfite (CaSO.sub.3.1/2H.sub.2 O), gypsum (CaSO.sub.4.2H.sub.2 O), calcium chloride (CaCl.sub.2) and calcium fluoride (CaF.sub.2). When desired, forced oxidation of the slurry by aeration is employed to ensure that all of the sulfites will be reacted to form sulfates, and thereby maximize the production of gypsum.
Known gas-liquid contactors typically include an absorber tower equipped with an inlet duct through which combustion gases enter the tower. Above the inlet duct is a bank of spray headers which introduce a contact medium, e.g., an alkaline slurry, into the tower. Additional banks of spray headers are often provided above the first bank of spray headers, as required for a given application. One or more pumps are required to recycle the alkaline slurry by pumping the slurry from a tank at the bottom of the tower to the spray headers. Intimate contact between the contact medium and the flue gases rising through the tower results in a cleansing action, after which the slurry and the entrapped or reacted gases are collected in the tank at the bottom of the tower. The cleansed gases continue to rise through the tower, then typically pass through a mist eliminator and thereafter are either heated or passed directly to the atmosphere through a chimney.
The tower of a conventional gas-liquid contactor typically has a large diameter, so that flue gas velocities through the tower are relatively low, typically less than twelve feet per second (about 3.7 m/s). Such low gas velocities are necessary to accommodate the inability of conventional mist eliminators to remove liquid out of a gas stream at higher velocities. However, higher flue gas velocities through an absorber tower would be advantageous for improving contact between the contact medium and the flue gases, which would reduce the amount of contact medium required for a given amount of flue gases in the tower. Higher flue gas velocities would also allow for the use of a tower having a smaller cross-sectional area, such that the cost of constructing the tower is reduced. However, flue gas velocities above 12 ft/s tend to increase the gas pressure drop within the tower, increasing the likelihood of liquid particles being carried to and flooding the mist eliminator.
In view of the above, it would be desirable if a flue gas scrubbing apparatus were available that was capable of operating at flue gas velocities above 12 ft/s to promote the efficiency of the absorption process, while overcoming the above-noted problems associated with high flue gas velocities.